1. Field of the Invention
The present invention relates to a laser light source device, and in particular to a laser light source device having reduced speckle of laser light emitted therefrom.
2. Description of the Related Art
Laser light source devices have conventionally been used as an illumination device for use with an endoscope or interferometer. Such an illumination device is required to provide uniform illumination light. However, an illumination device employing the laser light source device may suffer from degradation of illumination performance caused by an uneven illumination pattern (hereinafter referred to as “speckle”) due to coherence of the laser light. Further, in an imaging apparatus employing the laser light source device as the illumination device, the speckle of the laser light may significantly degrade quality of an acquired image.
Japanese Unexamined Patent Publication No. 2003-156698 proposes a laser light source device, in which the speckle of the laser light is reduced by applying vibration to an intermediate portion of a single-core multimode optical fiber. U.S. Pat. No. 7,113,657 proposes a laser light source device provided with a light phase controlling means, which controls the phase of laser light guided through a multimode optical fiber by forming a bend in the multimode optical fiber. Japanese Unexamined Patent Publication No. 6(1994)-167640 proposes a laser light source device, in which the speckle of the laser light is reduced by inputting the laser light to an optical fiber bundle formed by multimode optical fibers having fiber lengths that differ from each other by a length not less than the coherence length, and outputting the laser light from an output end of the optical fiber bundle.
However, since the speckle phenomenon is caused by the coherence of the laser light, if reduction of the coherence of the laser light is insufficient in the laser light source devices disclosed in the above Japanese Unexamined Patent Publication No. 2003-156698 and U.S. Pat. No. 7,113,657, the speckle reduction effect of the modulation applied to the mode coupling between multiple modes in the multimode optical fiber may not be visually recognizable. Namely, the present inventor has found that the speckle reduction effect of these laser light source devices is not visually recognizable if the laser light guided through the multimode optical fiber has a large amount of residual polarized component. The present invention has been achieved based on this finding.
Further, in the techniques disclosed in the above Japanese Unexamined Patent Publication No. 2003-156698 and U.S. Pat. No. 7,113,657, the modulation is applied to the multimode optical fiber by the mechanical driving means, and therefore, it is necessary to consider vibration and durability of the mechanical driving means in the laser light source device. In addition, in the technique disclosed in the above Japanese Unexamined Patent Publication No. 6(1994)-167640, it is difficult to produce the fiber bundle of fibers, which have fiber lengths that differ from each other by a length not less than the coherence length, and use of such a fiber bundle may lead to cost increase of the laser light source device.